1. Field of the Invention
The present invention relates to a method of determining the value of a received symbol obtained by demodulating a received digital modulated wave in a demodulator of a receiver used in a digital radio communications system to reproduce the symbol, and an apparatus destined for use in the demodulator of the receiver to carry out the method.
2. Description of the Prior Art
In the receivers such as a pager and the like used in conventional digital radio communications systems, the demodulator has a code reproducing circuit which compares with thresholds a received digital signal having been distorted over the transmission line to reproduce the digital signal. The conventional symbol determination apparatus to determine the value of a received symbol obtained through demodulation of the modulated wave of a received digital signal and reproduce the symbol, determines the symbol based on thresholds fixedly preset in the symbol determination apparatus. This will be explained with reference to a digital radio communications system utilizing a quaternary digital modulation, for example. This communications system may use four kinds of unit pulses or symbols as shown in FIG. 6. Signals formed from these four kinds of symbols, that is, symbols having values +1, +1/3, -1/3 and -1, respectively, will be transmitted over the communications system, and the signals received at a destination will result in eye patterns as shown in FIG. 7.
For the conventional determination of the above-mentioned value of such a signal having arrived (namely, received symbol), three kinds of thresholds, for example, +2/3, 0, -2/3, fixedly preset in a symbol determination apparatus are compared with the level of the received symbol as shown in FIG. 7. For example, when the level of the received symbol is larger than the threshold +2/3, the value of the received symbol is determined to be 1. When the level of the received symbol is smaller than the threshold 0 and larger than the threshold -2/3, the value of the symbol is determined to be -1/3 and the symbol is reproduced.
Assume here a conventional symbol determination apparatus designed to determine a symbol value through comparison of thresholds fixedly preset in the apparatus with the level of a received symbol. If a fading or a distortion due to a degradation of a transmission line in consideration causes a received symbol, namely, eye pattern, to be reduced in maximum amplitude as shown in FIG. 8, or if the eye pattern is deviated from a due one (shown in FIG. 7, for example) as shown in FIG. 9, the preset thresholds are not correctly applicable any longer for determination of the value of a received symbol, so that the noise margin is smaller, causing the signal reception to be degraded. In an extreme case, it is impossible to correctly determine the symbol whether or not the transmitted signal is affected by noises. The noises cause the symbol value to vary, resulting in the reduction in level of the symbol, so that the determination becomes erroneous.
A method of varying symbol is known from the disclosure in the Japanese Unexamined Patent Publication (Kokai) No. 60-174550. According to this disclosed invention, a mean value is taken from the levels of received symbols and a threshold is taken from the mean value to determine the value of a received symbol. With this prior-art technique, however, the symbol in a received data varies in level from one to another. A threshold derived by averaging the values of received symbols varying in level is not correctly applicable for determining the values of the received symbols. Also in this case, the noise margin is narrowed and the signal reception is degraded, so that no correct symbol determination is possible.
Accordingly, a symbol determination apparatus has so far been demanded which can be used to determine a received symbol with no degradation of signal reception due to a reduction of noise margin, etc. even if the received symbol is deteriorated due to a degradation of transmission line, etc.